1. Field of the Invention
The present invention relates generally to determination of the Packed Cell Volume or relative volume percent of erythrocytes (red blood corpuscles), also known as the hematocrit, of whole blood, and more specifically to a method and apparatus for making such determination noninvasively through coherent techniques.
2. State of the Art
Hematocrit is traditionally obtained by acquiring a patient blood sample from a vein via a syringe, or by use of a capillary tube from a finger stick, or puncture. The blood, contained in an elongated vessel, is then centrifuged and the height percentage of the column of blood in the vessel which is solid represents the hematocrit.
More recently, hematocrit has been obtained by the use of elaborate and expensive cell counting laboratory instruments which are also used to provide differentiations of white blood cells, platelets, etc. However, as with the centrifuge method, the blood must be invasively removed from the patient for analysis.
In the course of routine medical procedures, such as the daily blood work performed in hospitals, the necessity of obtaining blood samples from patients and then centrifuging or otherwise analyzing the drawn blood presents no great inconvenience, as the volume of samples is large (warranting expensive automated equipment) and the time delay in obtaining results from a laboratory is generally acceptable. However, in catastrophic situations such as are encountered in the emergency rooms and shock trauma units, as well as in the course of surgical procedures wherein blood loss is probable, the hematocrit determination apparatus and methodology of the prior art are markedly deficient.
In the foregoing environments, there may be no time to draw blood, and in fact it may be impossible to identify a vein from which to draw it. Drawing blood intermittently during surgical procedures is inconvenient if not impractical, and analyzing periodic samples is time and labor intensive. Moreover, hematocrit may vary and drop at such an accelerated rate from unobserved blood loss that by the time the emergency or surgical personnel are belatedly made aware of a problem by laboratory personnel, the patient may be in acute difficulty or even deceased.
It has been proposed to measure hematocrit noninvasively, as noted in "Noninvasive Measurement of Hematocrit by Electrical Admittance Plethysmography Technique," IEEE Transactions of Biomedical Engineering, Vol. BME-27, No. 3, March 1980 pp. 156-161. However, the methodology described in the foregoing article involves submerging an extremity, such as a finger, in an electrolyte (NaCl solution) and varying the electrolyte concentration to compensate for pulsatile electrical admittance variations by matching the electrolyte resistivity to that of the blood in the extremity; the resistivity of the electrolyte is then determined in a resistivity cell, and converted to a hematocrit value via a nonlinear least-squares regression calibration curve generated by matching centrifuged hematocrit for various erythrocyte concentrations to resistivity data previously taken directly from blood resistivity measurements of the same specimens. Aside from being unwieldy to employ in an emergency or operating room environment, to the inventors' knowledge the technique as described in the referenced article has never been followed up or verified by further research, or employed in practice.
A measurement technique termed "impedance plethysmography," or using impedance techniques to obtain a waveform, is conceptually rooted in biomedical antiquity. Medical literature abounds with vascular studies, respiration studies and attempts to determine cardiac output (the actual volume of blood flowing from the heart) by impedance techniques. None of these techniques has been proven to work particularly well, although there have been attempts at commercial instruments based on the concept. A variant of impedance plethysmography, however, electrically models intracellular as well as extracellular tissue components and employs a comparison of measurements of tissue impedance responsive to applied electrical currents at two frequencies to quantify the intracellular and extracellular tissue components. While not directly related to the problem solved by the present invention, the electrical tissue model is useful to an understanding thereof.
In recent years, a technique known as pulse oximetry has been employed to measure blood oxygenation during induction of general anesthesia. While pulse oximetry does not provide a hematocrit indication, one may consider it helpful to an understanding of the method and apparatus of the present invention. Pulse oximetry relies upon the fact that the light absorbance of oxygenated hemoglobin and that of reduced hemoglobin differ at two wavelengths of light (generally red and near infrared) employed in an oximeter, and that the light absorbances at both frequencies have a pulsatile component which is attributable to the fluctuating volume of arterial blood in the patient body portion disposed between the light source and the detector of the oximeter. The pulsatile or AC absorbance response component attributable to pulsating arterial blood is determined for each wavelength, as is the baseline or DC component which represents the tissue bed absorbances, including venous blood, capillary blood, and nonpulsatile arterial blood. The AC components are then divided by their respective DC components to obtain an absorbance that is independent of the incident light intensity, and the results divided to produce a ratio which may be empirically related to SaO.sub.2, or oxygen saturation of the patient's blood. An excellent discussion of pulse oximetry may be found in "Pulse Oximetry," by K. K. Tremper et al., Anesthesiology, Vol. 70, No. 1 (1989) pp. 98-108.